4. Earnings per share (Details) - USD ($)	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share [Abstract]
Dilutive Securities, Effect on Basic Earnings Per Share, Options and Restrictive Stock Units	$ 878,000	$ 853,000

Amount of increase (decrease) to net income used for calculating diluted earnings per share (EPS), resulting from the assumed exercise of stock options or restrictive stock units (RSUs).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Name:	us-gaap_AmountOfDilutiveSecuritiesStockOptionsAndRestrictiveStockUnits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration